The present invention relates to diameter measuring method and specifically to non-contact measurement of the diameter of a round object.
Existing diameter gages can be broadly divided into contact and non contact types. The most desirable gage is a non-contact type which can measure the diameter without requiring to straddle the measured object. Prior art devices not straddling the object are of the contact type, for example U.S. Pat. Nos. 3,172,208; 3,656,239 and 4,700,484. All these patents utilize a wheel which comes in contact with a rotating object. The wheel has to be carefully aligned with the object to achieve high accuracy. These gages can only measure rotating objects. Most prior art optical gaging, which is non-contact, have to straddle the object, for example U.S. Pat. Nos. 3,922,094 and 4,880,991. There are a few non-contact methods which only require access to one side of the object, such as U.S. Pat. No. 4,473,750, however, they require an accurate distance and angle between the gage and the object and are not suitable to cover a wide range of curvatures. Curvature measurements using two parallel beams can only be used when beams are approximately at a constant angle to the surface of the cylinder. In a hand-held device, these angles can not be controlled.